1. Field of the Invention
This invention relates to a fixing apparatus applied to an image forming apparatus such as a copier or a printer, and particularly to a fixing roller applied to the fixing apparatus.
2. Related Background Art
In image forming apparatuses such as copiers adopting the electrophotographic method, a fixing apparatus for effecting a heating process on a sheet which is a recording medium such as recording paper or a transferring material bearing an unfixed toner image thereon to thereby fix the unfixed image on the recording medium is known as a typical example of heating apparatuses.
Such a fixing apparatus is provided with a fixing roller which is a heating member for heat-melting, for example, of the toner of the toner image on the sheet, and a pressing roller which is a pressing member brought into pressure contact with the fixing roller and rotated following the fixing roller, and is adapted to heat and press the sheet bearing the toner image thereon while passing the sheet between the fixing roller and the pressing roller to thereby effect a fixing process.
A fixing roller of the heater type is formed into a hollow cylindrical shape with a metallic core material as a base, and a heat generating member disposed along the central shaft of the fixing roller is held in the fixing roller by holding means. The heat generating member is, for example, a tubular heat generating heat such as a halogen lamp, and generates heat by a predetermined voltage being applied thereto. Since this halogen lamp is located along the central shaft of the fixing roller, the heat generated from the halogen lamp is uniformly radiated to the inner wall surface of the fixing roller, and the temperature distribution on the outer wall surface of the fixing roller becomes circumferentially uniform. The outer wall surface of the fixing roller is heated to a temperature (e.g. 150xc2x0 C. to 200xc2x0 C.) suited for the fixing process, and thereafter is maintained at that temperature. In this state, the fixing roller and the pressing roller are adapted to be rotated in opposite directions while being in pressure contact with each other, and nip therebetween the sheet bearing the toner image thereon. Consequently, in the pressure contact portion (hereinafter referred to also as the nip portion) between the fixing roller and the pressing roller, the toner on the sheet is melted by the heat of the fixing roller, and becomes fixed on the sheet by the pressure acting from the two rollers.
Also, as a heating apparatus, there has been proposed a fixing apparatus of the induction heating type utilizing high frequency induction as a heating source, as disclosed in Japanese Patent Application Laid-Open No. 59-33787. In this fixing apparatus of the induction heating type, coils which are magnetic flux producing means are concentrically disposed in a holloe cylindrical fixing roller formed of a metal conductor, and an induction eddy current may be created in the fixing roller by a high frequency magnetic field produced by a high frequency electric current being supplied to the coils, and the fixing roller itself may be caused to generate Joule heat by the skin resistance of the fixing roller itself. According to this fixing apparatus of the induction heating type, the fixing roller itself generates heat and therefore, electrothermal conversion efficiency is very much improved.
Now, to achieve the shortening of warm-up time in the fixing apparatus of the heater type and the fixing apparatus of the induction heating type, it is preferable that the thickness of the mandrel be small.
Particularly in the fixing apparatus of the induction heating type, it is so contrived that the fixing roller will be caused to generate heat by the skin resistance of the fixing roller itself, and even if the thickness of the fixing roller itself is great, the amount of generated heat will not be varied. Therefore, when the mandrel of the fixing roller is thick, even if the fixing apparatus is of the induction heating type, heat generating efficiency is rather reduced and it becomes difficult to obtain the effect of shortening the warm-up time. Therefore, when use is made of a mandrel formed of iron, nickel or the like which is a usually used ferromagnetic material, it is preferable that the thickness thereof be about 100 to 1000 xcexcm.
However, when in the fixing apparatuses of the heater type and the induction heating type described above, a metal such as thin-walled iron or nickel is used as the mandrel of the fixing roller and in order to secure the fixing property, the high pressure force weighting by the pressing roller is effected on the fixing roller and the fixing apparatus is left as it is for a long period, there is the possibility that the thin-walled fixing roller will be plastically deformed and the flexure of the roller will occur.
Even if the fixing roller is not plastically deformed, when the pressure weighting by the pressing roller is effected on the fixing roller, there is the possibility that the fixing roller will be elastically deformed to thereby cause temporary flexure and therefore, the transferring material may be wrinkled during the conveyance thereof or a bad image may be caused by the warping of the trailing end of the transferring material, and the distance between the coil or core which is a heating source and the fixing roller may become unstable and the heat generation distribution may become uncontrollable.
Further, when sheets are repetitively passed and heated between the fixing roller and the pressing roller, there is the possibility that such a factor as the deterioration by the heat will be added and the destruction of the fixing roller will occur.
It is an object of the present invention to provide a fixing roller of which the shortening of the warm-up time is achieved and yet the deformation is prevented, and a fixing apparatus having the same.
It is another object of the present invention to provide a fixing roller having a metal layer of which the thickness is not less than 0.1 mm and not more than 2 mm, the tensile strength is 490 MPa or greater and the yield point is 294 MPa or greater.
It is still another object of the present invention to provide a fixing apparatus having a fixing roller having a metal layer, and a back-up member forming a nip with the fixing roller, wherein a recording material bearing an unfixed image thereon is nipped and conveyed by the nip, the unfixed image is fixed on the recording material, and the metal layer of the fixing roller has a thickness not less than 0.1 mm and not more than 2 mm, tensile strength of 490 MPa or greater, and a yield point of 294 MPa or greater.
Further objects of the present invention will become apparent from the following description.